Which Mechanism of Excitation Transfer Works in Antenna Systems of Photosynthetic Bacteria among Forster, Hot Transfer, Superexchange and Exciton?

Abstract

According to the three dimensional structures of antenna systems which were recently elucidated by the electron crystallography and X-ray crystallography, the chromophores are located in variety of symmetry and distances. Especially donor and acceptor for the excitaton transfer in the neighbourhood are mostly in close distance. So far, experimental and theoretical analyses of the excitation transfer mechanism in antenna systems were made using either the exciton theory which holds true when the excitonic coupling between the chromophores is so strong that the electronic coherence is never disturbed by the perturbation of vibrations or the Forster theory which holds true when the excitonic coupling is so weak that the electronic coherence is mostly disturbed by vibrations. The former is called the strong coupling case and the latter the weak coupling case. However, it is not evident in which antenna systems the strong coupling case applies and the weak coupling case applies. It is also natural to consider that the inermediate coupling case is appropriate in some cases. Here, we mean for the intermediate coupling case that the interaction between donor and acceptor is not so strong as to form an exciton state but is still considerably strong that the excitation transfer takes place before attaining the thermal equilibrium in the excited state of donor. In this paper, we present the excitation transfer theory in the intermediate coupling case and discuss new features of the intermediate excitation transfer. Finally we give a method to diagnosis what excitation transfer mechanism is appropriate for each antenna system.